Many high cost hydraulic equipment items such as pumps, reservoirs, valves and cylinders have hydraulic fluid ports in the form of bosses, each consisting of an internally threaded hole with a chamfered face surface. The bosses are designed to mate with appropriate replaceable fittings, each fitting and boss forming an annular cavity in which a rubber O-ring is installed to form a seal. In aerospace applications, such seals must withstand repeated applications of 3000 psi (20,000 kPa) hydraulic pressure, and under certain conditions transient pressures with peaks of 4500 psi (30,000 kPa). These peaks tend to stress the fitting with respect to the boss and open extrusion gaps at the metal to metal interface therebetween. The extrusion gaps to which the rubber O-ring seals are normally exposed are very minute. However, the angularity tolerance between the threads of the boss and the boss face can result in sufficient gapping especially in larger sizes to drastically reduce the life of the O-ring seal in the boss. This is because rubber under high pressure acts almost like a fluid and if the extrusion gap to which it is exposed is sufficiently large, the rubber of the O-ring can be forced therethrough with resultant seal failure. Such failures currently account for a major percentage of the failures encountered in aircraft hydraulic systems.
Previous attempts to solve fitting to boss sealing problems are also discussed in U.S. Pat. No. 3,730,563. In that patent the O-ring was completely eliminated through design of a lip seal. Such fittings typically leak slightly in response to design over pressures but do not fail. However, aircraft mechanics look for any leakage as an indication of seal failure and therefor such fittings were not universally accepted. The problem was partially solved by the modification of the lip seal fitting and the addition of a backup ring and an O-ring as shown in U.S. Pat. No. 3,850,456. The backup ring only properly positioned the O-ring and did not reduce the gap extrusion problem.